SPECIFIC AIM: To study the association between barium in water supplies and dental caries, apatite structure and incorporation in human dental enamel. METHODOLOGY. A). Data from the Environmental Protection Agency indicates that in W. Dundee (Illinois) the level of barium in the drinking water is above accepted guidelines and is health hazard. The limited dental literature implies that barium would increase caries but this has not been shown in man. Therefore, an epidemiologic survey will be carried out on children living in W. Dundee where there is 4 mg/l of barium in the drinking water. This survey will measure levels of dental caries as deft, defs, DMFT and DMFS as well as enamel defects such as mottling, opacities and pigmentations. Marengo (Illinois) will be used as a control community. B). Teeth will be collected for barium analysis to determine the uptake of the element in enamel and dentine. By using a sequential analytical method on layers of dentine the concentrations of barium laid down in dentine with time will be assessed. C). Incorporation of barium in synthetic carbonated-apatite will be studied using chemical analysis, X-ray diffraction, and IR spectroscopy, to assess the effect the barium will have on carbonated-apatite structure. D). Using a rat model the effect of barium at concentrations of 0.0, 0.1, 1.0, 10 and 100 mg/l in drinking water will be tested pre and post eruptively. SIGNIFICANCE. Other elements of the alkaline earth group (IIa) of the periodic table (Mg, Ca, Sr) have all been shown to have a relationship to dental caries. The proposed work will define the role of barium in the carious process and indicate whether it is cariostatic or cariogenic.